The present invention relates to protective systems against overvoltage for devices for the transmission of binary data through electric lines.
In these devices, the data is conveyed at the sending or receiving end by the opening or closing of a relay. Because the associated installations are then connected to the line, their operation may be seriously damaged if the voltage in said line changes suddenly.
If lightning strikes any point on the line, or even if it strikes nearby, it causes an overvoltage wave which propagates from the point of impact to the two extremities. The connected installations in turn can be affected by this overvoltage and severe damage may be caused.
For this type of installation, ground contacts can be adapted to ensure discharge of the current. This protection remains inadequate, owing to the impedance that these ground contacts always present. When lightning strikes only one of the installations, this may cause a momentary voltage surge in the installation. A voltage differential is then produced between the line and the installation.
To avoid these drawbacks, it is proposed to develop a binary data transmission system using an electric line wherein the tramsitter and the receiver are both sufficiently insulated with respect to the line and to the installations to withstand voltages up to 100,000 volts. Under such conditions, the voltage of the line is immaterial. It may be grounded or not.